Fuses are widely used as overcurrent protection devices to prevent costly damage to electrical circuits. Fuse terminals typically form an electrical connection between an electrical power source and an electrical component or a combination of components arranged in an electrical circuit. One or more fusible links or elements, or a fuse element assembly, is connected between the fuse terminals, so that when electrical current through the fuse exceeds a predetermined limit, the fusible elements melt and open one or more circuits through the fuse to prevent electrical component damage.
Circuit breakers also are widely used as overcurrent protection devices. A circuit breaker is an automatically-operated electrical switch. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operations. A circuit breaker panel is a mounting enclosure for multiple electrical circuit breakers connected to electrical circuits via one or more hot bus bars. Similarly, a fuse box is a mounting enclosure for multiple fuses. For simplicity, the term “panelboard” is used herein to refer to any mounting enclosure for multiple circuit breakers and/or fuses, including any circuit breaker panel and/or fuse box.
In 2005, the National Electrical Code (“NEC”) was revised to include requirements for selective coordination in emergency and legally required standby systems. The NEC defines “selective coordination” as the “localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings.” In other words, selective coordination involves isolating an overloaded or faulted circuit from the remainder of the electrical system by having only the nearest upstream overcurrent protective device open. The objective of the new selective coordination requirements is to ensure system uptime with the goal of safety of human life during emergencies or for essential health care functions, Selectively coordinating overcurrent protective devices is achieved by reviewing the operating characteristics of the upstream overcurrent protective devices in relation to the downstream overcurrent protective devices.
A traditional approach to selective coordination is providing a “fusible panelboard” with both circuit breakers and fuses having different overcurrent voltage ratings. Each branch circuit coupled to the fusible panelboard is associated with a different circuit breaker-fuse pair. Each circuit breaker provides on/off switching functionality to its respective branch circuit. The fuses associated with each branch circuit may be selected based on the selective coordination ratios provided by the fuse manufacturers Fusible panelboards generally have higher overcurrent interrupter ratings than conventional circuit breaker panelboards and fuse boxes.
However, fusible panelboards have had commercial difficulty due, at least in part, to the fact that existing fusible panelboards are significantly wider in size than ordinary circuit breaker panels and fuse boxes. The larger width is mostly due to the combination of a fuse holder and circuit breaker for each branch circuit. For example, an existing fusible panelboard is generally about 28 inches wide, as compared to a 20 inch width of a typical circuit breaker panelboard.
Therefore, a need exists in the art for a fusible panelboard having a decreased width.